Pipeline is nothing but the flow of different components in Informatica.A mapping in Informatica may contain Source, Transformations , Targets connected together, all these together will be considered as a pipeline. There will be many such pipelines in a single mapping .
One pipeline can be connected to another pipeline also; in that case it will be considered as a single pipeline
As seen in the below figure, Source, Transformations, Target are there in a pipeline.
To order the pipeline, there is a option in Designer i.e. Mappings>Target> Load Order